An Urban Heat Island (UHI) is an urbanized area with a greater temperature than the surrounding rural areas as a result of human activity. The temperature difference is most noticeable at night because the infrastructure materials absorb heat and radiation from the sun during the day and then release it during the night.[1] There are different factors that contribute to UHI, some controllable and some uncontrollable. Examples of controllable factors include anthropogenic heat, air pollutants, sky view factors, green areas, and building materials.[2] Examples of uncontrollable factors include changes in cloud cover, wind, and seasons.[3]
UHI effect is a significant problem and expected to increase over time. It causes deterioration of the environment, an increase in ground-level ozone elevation, an increase in energy consumption, and an impact on the death rate.[4] UHI effect is a more recent concern and due to the negative effect on the environment and population, much research and effort has gone towards how to prevent it. Luke Howard was the first to research and classify an Urban Heat Island.[5]
UHI Development
Luke Howard was the first person to research and describe UHI, even though he does not have his name with it. He was a meteorologist that focused extensively on clouds. From his research on clouds, he noticed the changes in temperatures of rural areas compared to the city (in his case: London).
As an area grows from rural to urban, there is an increase in infrastructure in a certain area. For example, land that was once covered by vegetation or forests is replaced with pavement, roads, and buildings. Changes like this directly effect the surrounding environment. Materials of an urban area absorb large amount of heat and radiation.
Sketch of an Urban Heat Island Profile
[6] In addition to an increase in infrastructure, there
is also an increase in population; this leads to higher energy consumption. The main heat source of UHI in anthropogenic heat, this includes heat sources created by the human population such as car emissions, high population densities, power plants, and air conditioning.[7] The larger the urban area, the larger the UHI is. As the United States has urbanized dramatically in the past few decades, UHIs have grown in size and number. The picture to the left shows how the temperature increases in the urban areas compared to the rural areas.
UHI Effects
Urban heat islands affect the amount of energy consumption. Increased temperature and intensified heat waves causes energy consumption to increase in order to manage the elevated temperatures.[8] UHIs can also affect human health. The effects include possible discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and even heat-related mortality.[9] The Centers for Disease Control and Prevention estimated that between 1979-2003, excessive heat exposure contributed to more than 8,000 premature deaths in the United States.[10] UHIs also increase the emissions of greenhouse gases and air pollutants.[11] Because of the increase in energy consumption is directly related to power plant, more greenhouse gases and air pollutants are released into the atmosphere.
Eliminating UHI
Three main ways that UHIs can be prevented are by increasing green vegetation, increasing urban albedo, and adjusting construction applications.
Increasing Green Vegetation
There have been studies analyzing the relationship between green vegetation and local temperatures. One study was done by the Energy and Environment Division conducted in the Whittier Narrows Recreation Area. The research measured and compared temperatures in vegetated urban areas and areas with vegetated canopies (such as dense trees).[12] Urban areas with vegetation could lower the temperatures by 34°F and vegetated canopies could decrease temperatures by 36°F.[13] The decrease in temperature is due to the extra shade, protection from wind, and evapotranspiration that the canopies provide.[14]
Increasing Urban Albedo
Urban albedo is the ability for an urbanized area to reflect sunlight. In many urban areas, roughly 60% of the cities are covered by pavement or rooftops.[15] Increasing the albedo is possible by covering these surface areas with lighter colors or increasing the reflectivity of the roofs.[16] The Energy and Environment Division conducted a study that compared urban areas with albedo and without albedo. The results showed that a reasonable increase in urban albedo could decrease the area’s temperature up to 36°F.[17]
Minor Adjustments
There are still numerous effective methods that have been presented to address UHI both directly and indirectly. Some of these methods include: minimal running of air conditioning units, creating more open air areas, running water over roofs, or proper building ventilation.[18] Not only do these solutions help reverse heat-island effects, but they also decrease the amount of energy consumed by the public. A decrease in energy consumption would create large financial benefits both locally and nationally. [19]
Construction Applications
Many improvements to the construction process have been presented to reduce the effects of UHI's. Most have already been discussed or mentioned earlier, but a list of construction applications includes:
Organic roofs are one wayto increase urban albedo.
Whitewashing or painting pavements and surfaces with reflecting colored paint.[20]
Using lighter colored material for laying concrete.[21]
Designing wider buildings in contrast to tall slim ones.[22]
Installing cooling units for pavements and roofs[25]
Increasing Sustainability
Since UHIs are a recently recognized problem, significant measurable data takes time to acquire so there are no reliable constructed works displaying sustainability. However, numerous tests and studies have been done which promise usable results. In 2005, a study was conducted to analyze the effectiveness of reflective surface materials and vegetation cover. Six places in New Jersey were chosen using a geographic information system modeling application. Models were computed in order to measure the initial UHI condition and then the effect of the two mitigation tactics (reflective surface materials and vegetation cover). The models were analyzed and data was collected over a 15-year span. Results showed the reflective surface materials were not very effective compared to vegetation cover. Increasing the amount of vegetation cover in an urban area decreased energy use and costs. As the vegetation grew, energy use and costs also decreased.[26]
Local Research
In 2011, the University of Texas in Arlington measured the UHI of the Dallas-Fort Worth area. Hourly atmospheric temperatures were collected in two areas – one in downtown Dallas and one in rural Kaufman (36 miles downwind). Temperature results showed that there was a 38°F difference between the two areas.[27] The results were compared to satellite imagery that provided land cover data as well as surface temperature. The results showed an increased surface temperature in the downtown Dallas area, which furthered proved the magnitude of the UHI effect in the Dallas-Fort Worth area. The images below show the type of land cover (left) and the surface temperatures (right).
Image showing land cover types.
Image showing increasedsurface temperature in urban areas.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Kum, Dan. (1994). The Potential for Reducing Urban Air Temperatures and Energy Consumption Through Vegetative Cooling. Berkeley, California.
^ Kum, Dan. (1994). The Potential for Reducing Urban Air Temperatures and Energy Consumption Through Vegetative Cooling. Berkeley, California.
^ Rosenfeld, A. H., (1998). “Cool communities: Strategies for heat island mitigation and smog reduction.” Energy and Buildings, 28(1). 1.
^ Akbari, Hashem. (2007). “Global Cooling: Effect of Urban Albedo on Global Temperature.” Lawrence Berkeley National Laboratory. <http://www.escholarship.org/uc/item/0pz748p6> (Nov. 27, 2014.)
^ Akbari, Hashem. (2007). “Global Cooling: Effect of Urban Albedo on Global Temperature.” Lawrence Berkeley National Laboratory. <http://www.escholarship.org/uc/item/0pz748p6> (Nov. 27, 2014.)
^ Taha, Haider. (1997). “Urban climates and heat islands: albedo, evapotranspiration, and anthropogenic heat.” Energy and Buildings. 25(1997). 99-103.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rosenfeld, A. H., (1998). “Cool communities: Strategies for heat island mitigation and smog reduction.” Energy and Buildings, 28(1). 1.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
^ Kum, Dan. (1994). The Potential for Reducing Urban Air Temperatures and Energy Consumption Through Vegetative Cooling. Berkeley, California.
^ Rizwan, Ahmed. (2007). “A review on the generation, determination and mitigation of Urban Heat Island.” Journal of Environmental Science. 20(2008). 120-122.
Table of Contents
UHI effect is a significant problem and expected to increase over time. It causes deterioration of the environment, an increase in ground-level ozone elevation, an increase in energy consumption, and an impact on the death rate.[4] UHI effect is a more recent concern and due to the negative effect on the environment and population, much research and effort has gone towards how to prevent it. Luke Howard was the first to research and classify an Urban Heat Island.[5]
UHI Development
Luke Howard was the first person to research and describe UHI, even though he does not have his name with it. He was a meteorologist that focused extensively on clouds. From his research on clouds, he noticed the changes in temperatures of rural areas compared to the city (in his case: London).
As an area grows from rural to urban, there is an increase in infrastructure in a certain area. For example, land that was once covered by vegetation or forests is replaced with pavement, roads, and buildings. Changes like this directly effect the surrounding environment. Materials of an urban area absorb large amount of heat and radiation.
[6] In addition to an increase in infrastructure, there
is also an increase in population; this leads to higher energy consumption. The main heat source of UHI in anthropogenic heat, this includes heat sources created by the human population such as car emissions, high population densities, power plants, and air conditioning.[7] The larger the urban area, the larger the UHI is. As the United States has urbanized dramatically in the past few decades, UHIs have grown in size and number. The picture to the left shows how the temperature increases in the urban areas compared to the rural areas.
UHI Effects
Urban heat islands affect the amount of energy consumption. Increased temperature and intensified heat waves causes energy consumption to increase in order to manage the elevated temperatures.[8] UHIs can also affect human health. The effects include possible discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and even heat-related mortality.[9] The Centers for Disease Control and Prevention estimated that between 1979-2003, excessive heat exposure contributed to more than 8,000 premature deaths in the United States.[10] UHIs also increase the emissions of greenhouse gases and air pollutants.[11] Because of the increase in energy consumption is directly related to power plant, more greenhouse gases and air pollutants are released into the atmosphere.
Eliminating UHI
Three main ways that UHIs can be prevented are by increasing green vegetation, increasing urban albedo, and adjusting construction applications.
Increasing Green Vegetation
There have been studies analyzing the relationship between green vegetation and local temperatures. One study was done by the Energy and Environment Division conducted in the Whittier Narrows Recreation Area. The research measured and compared temperatures in vegetated urban areas and areas with vegetated canopies (such as dense trees).[12] Urban areas with vegetation could lower the temperatures by 34°F and vegetated canopies could decrease temperatures by 36°F.[13] The decrease in temperature is due to the extra shade, protection from wind, and evapotranspiration that the canopies provide.[14]Increasing Urban Albedo
Urban albedo is the ability for an urbanized area to reflect sunlight. In many urban areas, roughly 60% of the cities are covered by pavement or rooftops.[15] Increasing the albedo is possible by covering these surface areas with lighter colors or increasing the reflectivity of the roofs.[16] The Energy and Environment Division conducted a study that compared urban areas with albedo and without albedo. The results showed that a reasonable increase in urban albedo could decrease the area’s temperature up to 36°F.[17]Minor Adjustments
There are still numerous effective methods that have been presented to address UHI both directly and indirectly. Some of these methods include: minimal running of air conditioning units, creating more open air areas, running water over roofs, or proper building ventilation.[18] Not only do these solutions help reverse heat-island effects, but they also decrease the amount of energy consumed by the public. A decrease in energy consumption would create large financial benefits both locally and nationally. [19]Construction Applications
Many improvements to the construction process have been presented to reduce the effects of UHI's. Most have already been discussed or mentioned earlier, but a list of construction applications includes:Increasing Sustainability
Since UHIs are a recently recognized problem, significant measurable data takes time to acquire so there are no reliable constructed works displaying sustainability. However, numerous tests and studies have been done which promise usable results. In 2005, a study was conducted to analyze the effectiveness of reflective surface materials and vegetation cover. Six places in New Jersey were chosen using a
geographic information system modeling application. Models were computed in order to measure the initial UHI condition and then the effect of the two mitigation tactics (reflective surface materials and vegetation cover). The models were analyzed and data was collected over a 15-year span. Results showed the reflective surface materials were not very effective compared to vegetation cover. Increasing the amount of vegetation cover in an urban area decreased energy use and costs. As the vegetation grew, energy use and costs also decreased.[26]
Local Research
In 2011, the University of Texas in Arlington measured the UHI of the Dallas-Fort Worth area. Hourly atmospheric temperatures were collected in two areas – one in downtown Dallas and one in rural Kaufman (36 miles downwind). Temperature results showed that there was a 38°F difference between the two areas.[27] The results were compared to satellite imagery that provided land cover data as well as surface temperature. The results showed an increased surface temperature in the downtown Dallas area, which furthered proved the magnitude of the UHI effect in the Dallas-Fort Worth area. The images below show the type of land cover (left) and the surface temperatures (right).
References